The roles of spinal adenosine receptors in the control of acute and more persistent nociceptive responses of dorsal horn neurones in the anaesthetized rat.

1. We describe here the effects of intrathecal selective adenosine receptor agonists on acute and more persistent evoked responses of dorsal horn nociceptive neurones recorded in intact rats anaesthetized with halothane. 2. The effects of the A1 receptor agonist, N6-cyclopentyladenosine and the non-selective agonist 2-chloroadenosine as well as the A2a receptor agonist, 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride were gauged on the C-, A delta-, A beta-fibre, post-discharge and wind-up responses produced by peripheral tanscutaneous stimulation. The antagonists, theophylline and 8(p-sulphophenyl) theophylline were also tested alone and to reverse the agonist effects. 3. Subcutaneous formalin (5%) was used to produce a more prolonged nociceptive response initiated by peripheral inflammation. 4. Both N6-cyclopentyladenosine and 2-chloroadenosine produced inhibitions of the C-fibre evoked responses, wind-up and post-discharge of the neurones with no significant effects on the A beta responses. By contrast, the A delta evoked responses were facilitated over the same time course and dose-range as the inhibitions. N6-cyclopentyladenosine was more potent and effective than 2-chloroadenosine. In marked contrast to these agonists, the A2a agonist produced only weak non-specific inhibitions. Theophylline and 8(p-sulphophenyl) theophylline alone had no effect on the acute responses but prevented or reversed inhibitory effects of N6-cyclopentyladenosine. 5. The formalin response was markedly inhibited by spinal N6-cyclopentyladenosine with both the acute first phase and more prolonged second phase being dose-dependently inhibited. N6-cyclopentyladenosine was considerably more potent on the formalin response than on the other neuronal measures. 6 The results suggest a role of adenosine Al receptors in the modulation of both acute and inflammatory nociception in the spinal cord.